1. Field of the Invention
The present invention relates to thin film photoelectric conversion devices and methods of manufacturing the same and in particular to reducing the cost of the same and improving the performance of the same. It should be noted that in the present specification the terms "polycrystalline", "microcrystalline" and "crystalline" are intended to also mean partially amorphous state, as normally used in the technical field of thin film photoelectric conversion devices.
2. Description of the Related Art
An amorphous silicon based solar cell is a typical thin film photoelectric conversion device. An amorphous photoelectric conversion material is usually prepared by a plasma CVD method at a film-forming temperature as low as about 200.degree. C., so that it can be formed on an inexpensive substrate, e.g., of glass, stainless steel, organic film, and thus expected as a preferable material for low-cost photoelectric conversion devices. Furthermore, since amorphous silicon has a large absorption coefficient for the visible-light range, a solar cell using an amorphous photoelectric conversion layer of a thickness less than 500 nm has achieved a short-circuit current more than 15 mA/cm.sup.2.
However, an amorphous silicon based material has a photoelectric conversion characteristic which tends to be disadvantageously degraded when it is irradiated with light for a long time period, as is referred to as the Stebler-Wronskey effect. Furthermore, its effective optical sensitivity range is limited to about 800 nm at the longer wavelength side. Therefore, a photoelectric conversion device using the amorphous silicon based material is rather limited in reliability, performance and the like, and its advantages, i.e., the high degree of freedom in substrate selection, the capability of applying low-cost process, and the like, are not fully made use of
In contrast, in recent years a photoelectric conversion device employing a thin film containing crystalline silicon such as polycrystalline silicon or microcrystalline silicon, has been increasingly developed. It has been developed in attempting to reduce the cost of the photoelectric conversion device and also enhance the performance of the same by forming a crystalline silicon thin film of good quality on an inexpensive substrate through a process at a low temperature, and such development is expected to be applied to a variety of photoelectric conversion devices, such as optical sensors other than solar cells.
Such crystalline silicon thin films may be formed, e.g., by employing chemical vapor deposition (CVD), sputtering or the like to deposit the same directly on a substrate or by employing a similar process to initially deposit an amorphous film and then thermally annealing or laser-annealing the film to achieve crystallization thereof. At any rate, it should be processed at most 550.degree. C. if such an inexpensive substrate as described above is used.
Among such processes, the technique of employing a plasma CVD method to directly deposit a crystalline silicon thin film most readily allows the process to be performed at low temperatures and the thin film to be increased in area, and is also expected to relatively readily provide a film of high quality. In employing such technique to obtain a polycrystalline silicon thin film, a crystalline silicon thin film of high quality is initially formed on a substrate through some process and then it can be used as a seed layer or a crystallization control layer so as to form thereon a polycrystalline silicon thin film of good quality even at relatively low temperatures.
It is also well known that a silane-type material gas may be diluted no less than 10 times with hydrogen and a pressure of 10 mTorr to 1 Torr may also be set in a plasma reaction chamber when a plasma CVD method is employed in film deposition to obtain a microcrystalline silicon thin film. In this example, a microcrystalline silicon thin film can readily be obtained at a temperature around 200.degree. C. For example, a photoelectric conversion device including a photoelectric conversion unit comprised of a p-i-n junction of microcrystalline silicon is described in Appl. Phys. Lett., Vol. 65, 1994, p. 860. This photoelectric conversion unit is formed of a p-type semiconductor layer, an i-type semiconductor layer as a photoelectric conversion layer and an n-type semiconductor layer which are successively simply deposited by a plasma CVD method and it is characterized in that these semiconductor layers are all of microcrystalline silicon. However, if a crystalline silicon film of high quality and hence a silicon-based thin-film photoelectric conversion device of high performance are formed according to conventional manufacturing methods, conditions and the like, the film deposition rate is as low as less than 0.6 .mu.m/hr in the thickness direction, equal to or lower than a deposition rate of an amorphous silicon film.
Japanese Patent Laying-Open No. 4-137725 describes that a low-temperature plasma CVD method is employed at a relatively high pressure of 5 Torr to form a silicon film. This example is, however, a comparative example with respect to the invention disclosed in Japanese Patent Laying-Open No. 4-137725, describing deposition of a silicon thin film directly on a substrate, e.g., of glass. The obtained film in the comparative example has low quality, not applicable to a photoelectric conversion device.
In general, when a plasma CVD method is employed at high pressures, a large amount of powdery products, dust and the like are produced in the plasma reaction chamber. Consequently, it is highly possible that the dust and the like fly onto a surface of the growing film and are thus taken into the growing film. As a result, the dust and the like may create pinholes in the film. To reduce such degradation in film quality, the reaction chamber must be cleaned frequently. The increased pressure in the reaction chamber renders such problems more significant particularly when a temperature lower than 550.degree. C. is used as a film deposition condition. In addition, in manufacturing a photoelectric conversion device such as a solar cell, it is necessary to deposit a thin film of large area and thus the increased pressure tends to disadvantageously reduce product yield and increases the labor and cost for maintenance of the film forming apparatus.
Accordingly, conventionally a pressure less than 1 Torr has typically been used, as described above, in employing a plasma CVD method to manufacture a thin film photoelectric conversion device.
A polycrystalline-type photoelectric conversion device including a crystalline silicon-based thin-film photoelectric conversion layer as described above has the following disadvantage: whether polycrystalline silicon or microcrystalline silicon partially containing amorphous phase is used as a photoelectric conversion layer of a solar cell, the silicon layer used is required to have a thickness as much as several micrometers to several tens micrometers to sufficiently absorb sunlight, considering the absorption coefficient of the crystalline silicon. It is thicker than an amorphous silicon photoelectric conversion layer by as much as close to one digit to two digits.
When according to conventional technology a plasma CVD method is used at low temperatures to obtain a crystalline silicon based thin film of good quality, the film grows at a rate approximately equal to or lower than a growth rate of an amorphous silicon film, e.g., approximately only 0.6 .mu.m/hr, even with temperature, pressure in the reaction chamber, RF power, gas flow rate and other various parameters for film-forming conditions that are preferably determined. In other words, the film deposition time required for forming a crystalline silicon thin film photoelectric conversion layer is several times to several tens times longer than that required for forming an amorphous silicon photoelectric conversion layer. This will render it difficult to enhance the throughput in the process for manufacturing the photoelectric conversion device and thus prevent the cost of the same from being reduced.